Objective To study the condition of using smart mobile device in junior primary school students of Changning district,Shanghai. Methods Cluster sampling was used to conduct a questionnaire survey on students of the first and second grade from two schools in Changning district.The students were surveyed about the condition of using smart mobile device. Results The rate of smart mobile device usage in junior primary school students was 83.50%.The major type was tablet computer or smart phone,most students used it at home,and the average use age was 2.83 years.The main use time everyday was less than 1 hour in the workday,while within 2 hours on the weekend.Mostly rest was once irregularly or within 1 hour.The primary purpose of using smart mobile device was playing games or searching informations.The intention was different between different genders (χ²=22.323,P<0.001).Most students used smart mobile device by the way of negotiation with the parents or within the fixed time.The arrange was different between different grades (χ²=13.489,P=0.004).The rate of addiction to smart mobile device in junior primary school students was 1.89%,with boys higher than the girls (χ²=7.672,P=0.006). Conclusion The application of smart mobile device in junior primary school students of Changning district is rather widespread,parents and the society should pay more attention.